Sankar Prasad Rath

A Remarkably Bent Diiron(III)-μ-Hydroxo Bisporphyrin: Unusual Stabilization of Two Spin States of Iron in a Single Molecular Framework

A novel diiron(III) bisporphyrin bridged by a hydroxo group between two cofacial Fe centers is reported. X-ray structural characterization revealed the remarkably bent μ-hydroxobis[Fe(III) porphyrin] with the smallest known Fe-O(H)-Fe angle [142.5(2)°] reported to date in an iron porphyrin. The close approach of the two rings in the molecule results in an unequal core deformation, and as a result, the geometrical parameters (such as the Fe-N(p), Fe-O and Fe···Ct(p) distances) are all different for the two Fe(III) centers, leading for the first time to a natural way of stabilizing two different spins of iron in a single molecular framework with complete retention of their own spectroscopic identities in both the solid state and solution. The strong antiferromagnetic coupling between the two Fe(III) centers in the μ-oxo dimer (-J = 126.6 cm(-1)) is attenuated to only 4.5 cm(-1) simply by protonation to give the μ-hydroxo complex.

Encapsulation of TCNQ and the Acridinium Ion within a Bisporphyrin Cavity: Synthesis, Structure, and Photophysical and HOMO–LUMO‐Gap‐Mediated Electron‐Transfer Properties

The encapsulation of tetracyanoquinodimethane (TCNQ) and fluorescent probe acridinium ions (AcH(+)) by diethylpyrrole-bridged bisporphyrin (H(4)DEP) was used to investigate the structural and spectroscopic changes within the bisporphyrin cavity upon substrate binding. X-ray diffraction studies of the bisporphyrin host (H(4)DEP) and the encapsulated host-guest complexes (H(4)DEP⋅TCNQ and [H(4)DEP⋅AcH]ClO(4)) are reported. Negative and positive shifts of the reduction and oxidation potentials, respectively, indicated that it was difficult to reduce/oxidize the encapsulated complexes. The emission intensities of bisporphyrin, upon excitation at 560 nm, were quenched by about 65 % and 95 % in H(4)DEP⋅TCNQ and [H(4)DEP⋅AcH]ClO(4), respectively, owing to photoinduced electron transfer from the excited state of the bisporphyrin to TCNQ/AcH(+); this result was also supported by DFT calculations. Moreover, the fluorescence intensity of encapsulated AcH(+) (excited at 340 nm) was also remarkably quenched compared to the free ions, owing to photoinduced singlet-to-singlet energy transfer from AcH(+) to bisporphyrin. Thus, AcH(+) acted as both an acceptor and a donor, depending on which part of the chromophore was excited in the host-guest complex. The electrochemically evaluated HOMO-LUMO gap was 0.71 and 1.42 eV in H(4)DEP⋅TCNQ and [H(4)DEP⋅AcH]ClO(4), respectively, whilst the gap was 2.12 eV in H(4)DEP. The extremely low HOMO-LUMO gap in H(4)DEP⋅TCNQ led to facile electron transfer from the host to the guest, which was manifested in the lowering of the CN stretching frequency (in the solid state) in the IR spectra, a strong radical signal in the EPR spectra at 77 K, and also the presence of low-energy bands in the UV/Vis spectra (in the solution phase). Such an efficient transfer was only possible when the donor and acceptor moieties were in close proximity to one another.

Chemistry of hydrazonato oxovanadium(V) alkoxides derived from dihydric/monohydric alcohols

Abstract The reaction of bis(acetylacetonato)oxovanadium(IV) with benzoylhydrazones of benzoylacetone and salicylaldehyde — H2babh and H2sabh, respectively — with ethane-1,2-diol (H2ed) in acetone solution has afforded the alkoxides VVO(babh)(Hed) and VVO(sabh)(Hed) in which the Hed− ligand is chelated both in the solid state and in solution. In methanol solution (no H2ed added) five-coordinated VvO(babh)(OMe) and six-coordinated VVO(sabh)(OMe)(OHMe) are formed. Aerial oxygen is the oxidant (VIV → VV) in the synthesis. The X-ray structures of VO(babh)(Hed) and VO(sabh)(OMe)(OHMe) are reported. In the VO5N coordination sphere the alcohol oxygen lies trans to the oxo oxygen. The general VO bond length order is oxo E 1 2 , −0.3 to −0.2 V ) corresponding to stabilization of the pentavalent state. The methylene protons of Hed− in the complexes are inequivalent in solution (1H NMR). Crystal data: VO(babh)(Hed): monoclinic, space group P2 1 /c, a = 11.260(6), b = 7.672(3), c = 21.072(8) A , β = 91.95(4)°, V = 1820(1.4) A 3 , Z = 4, R w = 4.67% ; VO(sabh)(OMe)(OHMe): monoclinic, space group P2 1 /c, a = 8.170(4), b = 16.984(10), c = 12.242(7) A , β = 104.24(4)°, V = 1646(1.6) A 3 , Z = 4, R = 3.64% , Rw = 4.71%.

Effect of Heme–Heme Interactions and Modulation of Metal Spins by Counter Anions in a Series of Diiron(III)-μ-hydroxo Bisporphyrins: Unusual Stabilization of Two Different Spins in a Single Molecular Framework

A new family of five ethene-bridged diiron(III)-μ-hydroxo bisporphyrins with the same core structure but different counter anions, represented by the general formula [Fe2 (bisporphyrin)]OH·X (X=counter anion), is reported herein. In these complexes, two different spin states of Fe are stabilized in a single molecular framework. Protonation of the oxo-bridged dimer 1 by strong Brønsted acids such as HI, HBF4, HPF6, HSbF6 , and HClO4 produces the μ-hydroxo complexes with I5(-)(2), BF4(-)(3), PF6(-)(4), SbF6(-)(5), and ClO4(-)(6) as counter anions, respectively. The X-ray structures of 2 and 6 have been determined, which provide a rare opportunity to investigate structural changes upon protonation. Spectroscopic characterization has revealed that the two iron(III) centers in 2 are nonequivalent with nearly high and admixed-intermediate spins in both the solid state and solution. Moreover, the two different Fe(III) centers of 3-5 are best described as having admixed-high and admixed-intermediate spins with variable contributions of S=5/2 and 3/2 for each state in the solid, but two different admixed-intermediate spins in solution. In contrast, the two Fe(III) centers in 6 are equivalent and are assigned as having high and intermediate spin states in the solid and solution, respectively. The X-ray structures reveal that the Fe-O bond length increases on going from the μ-oxo to the μ-hydroxo complexes, and the Fe-O(H)-Fe unit becomes more bent, with the dihedral angle decreasing from 150.9(2)° in 1 to 142.3(3)° and 143.85(2)° in 2 and 6, respectively. Variable-temperature magnetic data have been subjected to a least-squares fitting using the expressions derived from the spin Hamiltonians H=-2JS1·S2 -μ·B+D[S(2)(z) - 1/3S(S + 1)] (for 2, 3, 4, and 5) and H=-2JS1·S2 (for 6). The results show that strong antiferromagnetic coupling between the two Fe(III) centers in 1 is attenuated to nearly zero (-2.4 cm(-1)) in 2, whereas the values are -46, -32.6, -33.5, and -34 cm(-1) for 3, 4, 5, and 6, respectively.

Efficient complexation of pyrrole-bridged dizinc(II) bisporphyrin with fluorescent probe pyrene: synthesis, structure, and photoinduced singlet-singlet energy transfer.

A diethylpyrrole-bridged dizinc(II) bisporphyrin (Zn(2)DEP) is reported that encapsulates fluorescent probe pyrene molecules through strong π-π interactions, which can relay information about the chemical environment in the interior of the host-guest supramolecular assembly. X-ray structures of both Zn(2)DEP and the encapsulated pyrene complex are reported, which provides a rare opportunity to investigate the structural changes upon guest binding. A comparative structural analysis demonstrated the exceptional ability of this bisporphyrin platform to open its binding pocket for pyrene encapsulation by a vertical displacement of more than 2.45 Å, although both Zn(2)DEP and the pyrene complex have nearly parallel porphyrin ring orientations. The (1)H NMR spectrum of the encapsulated pyrene complex in solution shows the upfield shifts of the pyrene protons due to a strong ring current effect, which demonstrates the retention of the solid-state structure in solution. To further assess the extent to which pyrene guests remain encapsulated in solution, a known fluorescence quencher, dimethylaniline, was added to the host-guest assembly, which shows no exciplex formation for days in nonpolar solvents. Thus, the assembly also retained the structural integrity in solution for a long time. The association constant (K(asso)) for such a complexation process in solution was observed to be 1.78×10(5) M(-2) for 1:2 binding. Steady-state fluorescence and lifetime studies indicate significant photoinduced singlet-singlet energy transformation from the excited state of pyrene to zinc bisporphyrin.

Supramolecular BODIPY-Zn(II)-bisporphyrin dyad and trinitrofluorenone encapsulated triad as models of antenna-reaction center: synthesis, structure and photophysical properties.

We describe the synthesis, characterization and photophysical properties of the dyads Zn2DEP·TNF (1) and Zn2DEP·2Py-BDP (2) and triad Zn2DEP·2Py-BDP·TNF (3), which enable us to investigate the changes in the spectroscopic properties upon the axial coordination of pyridine substituted bodipy (Py-BDP) to Zn2DEP in the dyads and the further encapsulation of TNF in the triad. Zn2DEP·TNF (1) is structurally characterized while the other compounds, Py-BDP, 2 and 3, are geometrically optimized using DFT in the absence of X-ray crystallography. It has been found that the encapsulated TNF guest within the bisporphyrin cavity is involved in strong π-π interactions with the host resulting in robust host-guest assemblies that also retain the structural integrity in solution for a long time. The association constants, K, between the host and guests are calculated at 298 K by measuring the change in the fluorescence emission in solution and are found to be 1.0 × 10(7) M(-2) for the 1 : 2 binding in 2 and 1.2 × 10(3) M(-1) for the 1 : 1 binding in 3. The solution (1)H NMR spectrum of the supramolecular dyad 2 shows an upfield shift of the Py-BDP protons due to the strong ring current of the porphyrin rings. Similarly, upon the encapsulation of TNF in triad 3, the guest protons undergo an upfield shift owing to the ring current effect. Electrochemical studies of dyad 2 indicate that the Py-BDP and Zn-bisporphyrin subunits in the dyads interact very weakly. The negative and positive shifts of the reduction and oxidation potentials indicate that it is difficult to reduce/oxidize the encapsulated complex 3. The steady state emission spectrum shows that the fluorescence intensity of the axially coordinated Py-BDP in 2 (excited at 500 nm) is remarkably quenched in comparison to free Py-BDP due to efficient photoinduced intramolecular singlet-to-singlet energy transfer from the excited state of Py-BDP to the zinc-bisporphyrin in 2. The remarkable quenching of the fluorescence intensity of Py-BDP upon excitation at 500 nm in 3 is due to photoinduced electron transfer from the excited states of Py-BDP to TNF, which is also thermodynamically favorable. The location of the electron densities on the HOMO and LUMO further support the formation of a charge-separated state in which the zinc-bisporphyrin acts as an electron donor whilst TNF acts as an electron acceptor in the supramolecular triad 3. The electrochemically evaluated HOMO-LUMO gap also maintains a good agreement with the computed HOMO-LUMO gap.

Protonation of an Oxo‐Bridged Diiron Unit Gives Two Different Iron Centers: Synthesis and Structure of a New Class of Diiron(III)‐μ‐hydroxo Bisporphyrins and the Control of Spin States by Using Counterions

Reported herein is a hitherto unknown family of diiron(III)-μ-hydroxo bisporphyrins in which two different spin states of Fe are stabilized in a single molecular framework, although both cores have identical molecular structures. Protonation of the oxo-bridged dimer (2) by using strong Brønsted acids, such as HI, HBF(4), and HClO(4), produce red μ-hydroxo complexes with I(3)(-) (3), BF(4)(-) (4), and ClO(4)(-) (5) counterions, respectively. The X-ray structure of the molecule reveals that the Fe-O bond length increases on going from the μ-oxo to the hydroxo complex, whereas the Fe-O(H)-Fe unit becomes more bent, which results in the smallest known Fe-O(H)-Fe angles of 142.5(2) and 141.2(1)° for 3 and 5, respectively. In contrast, the Fe-O(H)-Fe angle remains unaltered in 4 from the corresponding μ-oxo complex. The close approach of two rings in a molecule results in unequal core deformations in 3 and 4, whereas the cores are deformed almost equally but to a lesser extent in 5. Although 3 was found to have nearly high-spin and admixed intermediate Fe spin states in cores I and II, respectively, two admixed intermediate spin states were observed in 4. Even though the cores have identical chemical structures, crucial bond parameters, such as the Fe-N(p), Fe-O, and Fe⋅⋅⋅Ct(p) bond lengths and the ring deformations, are all different between the two Fe(III) centers in 3 and 4, which leads to an eventual stabilization of two different spin states of Fe in each molecule. In contrast, the two Fe centers in 5 are equivalent and assigned to high and intermediate spin states in the solid and solution states, respectively. The spin states are thus found to be dependent on the counterions and can also be reversibly interconverted. Upon protonation, the strong antiferromagnetic coupling in the μ-oxo dimer (J, -126.6 cm(-1)) is attenuated to almost zero in the μ-hydroxo complex with the I(3)(-) counterion, whereas the values of J are -36 and -42 cm(-1), respectively, for complexes with BF(4)(-) and ClO(4)(-) counterions.

Synthesis, structure, and properties of a series of chiral tweezer-diamine complexes consisting of an achiral zinc(II) bisporphyrin host and chiral diamine guest: induction and rationalization of supramolecular chirality.

We report here the synthesis, structure, and spectroscopic properties of a series of supramolecular chiral 1:1 tweezer-diamine complexes consisting of an achiral Zn(II) bisporphyrin (Zn2DPO) host and five different chiral diamine guests, namely, (R)-diaminopropane (DAP), (1S,2S)-diaminocyclohexane (CHDA), (S)-phenylpropane diamine (PPDA), (S)-phenyl ethylenediamine (PEDA), and (1R,2R)-diphenylethylene diamine (DPEA). The solid-state structures are preserved in solution, as reflected in their (1)H NMR spectra, which also revealed the remarkably large upfield shifts of the NH2 guest protons with the order Zn2DPO·DAP > Zn2DPO·CHDA > Zn2DPO·PPDA> Zn2DPO·PEDA ≫ Zn2DPO·DPEA, which happens to be the order of binding constants of the respective diamines with Zn2DPO. As the bulk of the substituent at the chiral center of the guest ligand increases, the Zn-Nax distance of the tweezer-diamine complex also increases, which eventually lowers the binding of the guest ligand toward the host. Also, the angle between the two porphyrin rings gradually increases with increasing bulk of the guest in order to accommodate the guest within the bisporphyrin cavity with minimal steric clash. The notably high amplitude bisignate CD signal response by Zn2DPO·DAP, Zn2DPO·CHDA, and Zn2DPO·PPDA can be ascribed to the complexs high stability and the formation of a unidirectional screw as observed in the X-ray structures of the complexes. A relatively lower value of CD amplitude shown by Zn2DPO·PEDA is due to the lower stability of the complex. The projection of the diamine binding sites of the chiral guest would make the two porphyrin macrocycles oriented in either a clockwise or anticlockwise direction in order to minimize host-guest steric clash. In sharp contrast, Zn2DPO·DPEA shows a very low amplitude bisignate CD signal due to the presence of both left- (dictated by the pre-existing chirality of (1R,2R)-DPEA) and right-handed screws (dictated by the steric differentiation at the chiral center) of the molecule, as evident from X-ray crystallography. The present work demonstrates a full and unambiguous rationalization of the observed chirality transfer processes from the chiral guest to the achiral host.

Axial Thiophenolate Coordination on Diiron(III)bisporphyrin: Influence of Heme–Heme Interactions on Structure, Function and Electrochemical Properties of the Individual Heme Center

The binding of a series of substituted thiophenols as axial ligands on a highly flexible ethane-bridged diiron(III)bisporphyrin framework has been investigated as a model of diheme proteins. Spectroscopic characterization reveals a high-spin (S = 5/2) state of iron for all of the pentacoordinate thiophenolato complexes. In the UV-visible spectra of the complexes, the positions of the Soret and band I have been found to be dependent on the pKa of thiophenols. The alternating shift pattern, which has opposite sign of the chemical shifts for meta- vs. ortho- and para-protons in the (1)H NMR spectra, is attributed to negative and positive spin densities, respectively, on thiophenolate carbon atoms and is indicative of π-spin delocalization to the bound thiophenolate ligand. The Fe(III)/Fe(II) redox couple of the complexes bears a linear relationship with the pKa of thiophenol and is found to be positively shifted with decreasing pKa. The effect of the electronic nature of the substituent on the thiophenolate ring has also been demonstrated in which a large potential range of 540 mV was observed (in contrast to the value of only 270 mV in case of monoheme analogues) for the Fe(III)/Fe(II) redox couple on going from monoheme to diheme and is attributed to the interheme interaction. Also, the Fe(III)/Fe(II) redox potential of the thiophenolato complexes has been found to be more positively shifted compared to their phenolato analogues, which was further supported by DFT calculation. The addition of another thiophenol at the sixth axial position of the five-coordinate thiophenolato complex causes a change in iron spin from high (S = 5/2) to low (S = 1/2) along with a large positive shift of 490 mV for the Fe(III)/Fe(II) redox couple.

Switching orientation of two axial imidazole ligands between parallel and perpendicular in low-spin Fe(III) and Fe(II) nonplanar porphyrinates.

We have reported here the synthesis, structure, and properties of low-spin bis-imidazole-coordinated Fe(III) and Fe(II) complexes of 5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octachloroporphyrin, [Fe(III)(TFPPCl(8))(L)(2)]ClO(4) and Fe(II)(TFPPCl(8))(L)(2) (L = 1-methylimidazole, 4-methylimidazole, imidazole). The X-ray structure of Fe(II)(TFPPCl(8))(1-MeIm)(2) is reported here, which demonstrated the near-perpendicular axial ligand orientation (dihedral angle between two 1-methylimidazoles is 80.7°) for Fe(II) porphyrins in a highly saddle-distorted macrocyclic environment. Oxidation of Fe(II)(TFPPCl(8))(L)(2) using thianthrenium perchlorate produces [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which was also isolated in the solid state and characterized spectroscopically. The complex gives rhombic EPR spectra in both solid and solution phases at 77 K and thus represents a rare example of nearly parallel axial ligand orientations for the unhindered imidazoles in a saddle-distorted porphyrin macrocycle. Geometry optimization using DFT also converged to the parallel axial alignment when 1-methylimidazole was used as the axial ligand (the dihedral angle between two axial ligands is 8.6°). The potential energy surface (PES) scan results also show that the relatively parallel axial orientations are energetically preferred for Fe(III), while perpendicular orientations are preferred for the Fe(II) complexes reported here. Bulk oxidation of Fe(II)(TFPPCl(8))(L)(2) in dichloromethane at a constant potential under nitrogen converts it to [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which gives identical EPR spectra at 77 K and which upon reduction regenerates Fe(II)(TFPPCl(8))(L)(2) again. Thus, we have demonstrated here very rare examples of Fe porphyrins in which the relative axial imidazole orientations switch between parallel and perpendicular just upon changing the oxidation states of iron from +3 to +2, respectively, in a nonplanar porphyrinic environment. These observations could be immensely important for understanding the possible effects of axial histidine orientations on similar macrocyclic deformations observed in various heme proteins.